rel|a|tiv|i|ty

rel|a|tiv|i|ty «_REHL uh TIHV uh tee», noun.

1. the condition of being relative.

2. Philosophy. existence only in relation to the human mind.

3. Physics. the character of being relative rather than absolute, as ascribed to motion or velocity.

4. a) a theory dealing with the physical laws which govern time, space, mass, motion, and gravitation, expressed in certain equations by Albert Einstein; special theory of relativity. According to it, the only velocity we can measure is velocity relative to some body, for if two systems are moving uniformly in relation to each other, it is impossible to determine anything about their motion except that it is relative, and the velocity of light is constant, independent of either the velocity of its source or an observer. Thus it can be mathematically derived that mass and energy are interchangeable, as expressed in the equation E = mc², where c = the velocity of light; that a moving object appears to be shortened in the direction of the motion to an observer at rest; that a clock in motion appears to run slower than a stationary clock to an observer at rest; and that the mass of an object increases with its velocity. »

One of the fundamental postulates of relativity is that the velocity of light is the same in all circumstances, even when the source and the observer are in relative motion (W. H. Marshall).

b) an extension of this theory, dealing with the equivalence of gravitational and inertial forces; general theory of relativity.